1. Field of the Invention
This invention relates to thrust augmentation in gas turbine engines, and more particularly to pilot stabilized combustion in a swirl augmentor.
2. Description of the Prior Art
Turbojet engines for aircraft use operatively produce a stream of gases exiting from the engine. The thrust generated by an engine is a function of the exhaust gas velocity and of the exhaust gas pressure. The higher the velocity and the higher the pressure of the exiting gases, the higher the corresponding thrust becomes. To obtain high velocities, fuel and high pressure air are burned in a combustion chamber within the engine. The high pressure air is supplied to the combustion chamber by a compressor upstream of the chamber. A predominant amount of the energy added to the medium gases in the combustion process is used to drive the compressor. The remaining portion of the added energy is convertible to engine thrust.
Early in the development of turboject engines and in response to the military need for high performance aircraft, a second combustion station was added downstream of the turbine section to augment the thrust contribution of the main combustor. At the second combustion station the velocity of the gases is increased by adding additional energy to the gases. Combustion at the second station has become commonly known as "augmenting" or "after-burning", reburning of the gases originally burned in the main combustor.
Techniques employed in afterburners were at their inception, and remain today, quite distinct from those employed in main combustors. In particular, combustion concepts directed to the stabilization of a flame front at each respective combustion station differ widely. In main combustors the aerodynamic effects of locally swirling gases are employed to stabilize the flame front at the desired location. U.S. Pat. No. 2,676,460 to Brown entitled "Burner Construction of the Can-Annular Type Having Means For Distributing Airflow to Each Can" is representative of swirl stabilized main combustors. In augmentors, on the other hand, bluff body flameholders are disposed across the path of the medium gases to induce recirculation of gases behind the flameholders. Recirculation of the gases stabilizes and holds the flame front at the proximate location of the flameholder. U.S. Pat. No. 2,702,452 to Taylor entitled "Flameholder Construction" is representative of early concepts for flameholder stabilization in augmentors.
The augmentor of Taylor is directed to a turbojet type gas turbine engine. Since the early 1960's, however, gas turbine engines of prime importance have been those based upon the turbofan cycle. In a turbofan cycle engine a substantial portion of the air flowing through the engine is caused to bypass the main combustor. At the augmentor, therefore, the gas stream is comprised of a central core stream of relatively hot gases from the main combustor and a surrounding bypass stream of relatively cool gases. Before effective combustion of the combined streams can be affected, the cold air stream must be heated as through mixing with the hot core gases. One widely accepted technique for mixing the gases is to shape the afterburner flameholder such that it causes hot gases of the core to be directed outwardly into the cold air stream. U.S. Pat. No. 3,295,325 to Nelson entitled "Jet Engine Afterburner Flameholder" illustrates such a shaped flameholder and describes its operation.
Very recent advances in combustor and augmentor technology are disclosed in U.S. Pat. No. 3,788,065 to Markowski entitled "Annular Combustion Chamber For Dissimilar Fluids in Swirling Flow Relationship" and in U.S. Pat. No. 3,747,345 to Markowski entitled "Shortened Afterburner Construction For Turbine Engine" which adapts the concepts of the U.S. Pat. No. 3,788,065 to augmentor embodiments. The concepts disclosed in these Markowski patents are now known in the industry as "swirl burning". Note in the U.S. Pat. No. 3,747,345 that, even with these most recent combustion techniques, the flame stabilization concepts of prior turbojet and turbofan augmentors are utilized.
Scientists and engineers in the gas turbine field are continuing to search for new stabilization concepts and techniques, and particularly those which can adapt swirl burning techniques to effective embodiments.